The present invention relates to an information recording medium, an information recording method and an information recording apparatus which are capable of recording such information in real time by means of recording beams, for example, laser beams as digital information including pulse-frequency modulated analog signals of projected images and voice, data in electronic computers, facsimile signals and digital audio signals. More particularly, the present invention relates to a disk shaped recording medium using an erasable, phase-change type optical recording film.
Conventional phase-change type optical disks have a recording film capable of fast erasure by crystallization that is materialized within a time substantially equal to the time required for a recording laser beam to pass on a point on a disk, for example. The conventional method of recording and erasing information to and from such an erasable recording film is to change the power of the laser beam between two levels higher than the reading power levels, these levels are a high power level and an intermediate power level. The advantage of this method is that the so-called overwriting (replacing old information with new without prior erasing) is possible. When such overwriting is repeated a plurality of times, however, the laser beam irradiates the recording film repeatedly in accordance with the recording information signal. This results in a high probability of allowing VFO recording marks to be repeatedly written to the same place in an area close to a preformat section in particular, and therefore variations in film thickness tend to occur locally because of recording film fluidization.
According to Japanese Unexamined Patent Publication No. 150725/1991, a method that avoids the recording film from being repeatedly written at the same place involves shifting the data write start position each time data is written or at appropriate times when old data is overwritten, whereby mark-forming positions are properly distributed. Thus the overwriting cyclability of the optical disk are improved.
Even when information is recorded on the phase-change type optical disk as in the prior art, the fluidization of the overall recording film inevitably occurs as overwriting is repeated a plurality of times because the temperature distribution in the laser beam is asymmetrical. Consequently, a change in the thickness of the recording film is brought about in that the film becomes thick on one side and thin on the other side (the initial or terminal end portion of a recording area taken from the perspective of the direction of recording) or the film becomes thick at the center portion of the recording area and thin at the starting and ending areas, so that the distortion of the reproducing signal waveform also occurs. The latter type of thickness change is more difficult to stop. Therefore, there arises a problem of making the area having the waveform distortion expand as the number of overwriting times increases. In the case of an erasable digital video disk, moreover, mark edge recording may be used with both edges of a recording mark provided with information for the purpose of increasing density. When such a mark edge recording system is employed, deterioration due to fluidization is further evident because a long mark is repeatedly recorded.
An object of the present invention intended to solve the foregoing problems in the prior art is to provide an information recording medium, an information recording method and an information recording apparatus that ensure reproduced signals properly corresponding to recorded information signals.
According to the present invention, a recording medium capable of recording or overwriting information with a laser beam is provided. The recording medium has a preformatted area in which information such as a track address has been formed with pits or with recording marks resulting from a phase change. In addition to the preformatted area, a gap area, a recordable area such as a VFO/SYNC area and a user data area are reserved. Moreover, a dummy area is provided on at least one of the sides before and after the recording area. A part of the VFO area can be used instead of the front dummy area, by making the VFO area long. Further, a buffer area to allow for a margin for the rotational jitter of a motor is located just after the dummy area. In this case, one of the objectives for the provision of the dummy area is to prevent information actually read from the user data area from being damaged even though repeated overwriting causes the fluidization of the recording film. Such damage is prevented because the distortion of a reproduced signal waveform is confined to the VFO/SYNC area and/or the dummy area. Further, the dummy area is preferably set to be longer than the maximum shift width of a position where the writing of a recorded information signal is started. Further, it is preferable to add a pit array (data area postamble) depending in accordance with applicable standards before this dummy area to avoid the mark and the space from being inverted.
According to the invention, recording is made by adding a dummy signal to at least the front or rear side of a target (recorded) signal such as VFO, SYNC and user data. It is preferable to add the dummy signal to the rear side (the trailing side with respect to the direction of recording) when a dummy signal is added to only one side of the target signal. In this case, the dummy signal is chiefly recorded in the dummy area.
Further, according to the invention, the fluidization of the recording film resulting from repeated overwriting can be moderated by setting the average energy of the laser beam during dummy signal recording to be lower than that during the target information signal recording. Consequently, the distortion of the reproduced signal waveform due to the fluidization slightly occurs in the dummy area where the dummy signal is recorded and in part of the buffer area. However, the recording area where the target signal is recorded remains unaffected by the fluidization to ensure that a reproduced signal correctly corresponding to the target signal is obtained. Collectively, the sum of the dummy area and the buffer area can be called a buffer area.
For the recording of the dummy signal, the average energy thereof may be lowered, for example, by adjusting the duty ratio of a single-frequency signal. In a mark edge recording system, the duty ratio of the dummy signal may be set to not greater than 50% because the average duty ratio is about 50%. The duty ratio of at least a part of the dummy signal is preferably in a range of at least 10% to about 40%, and more preferably in a range of at least 20% to about 30%. Moreover, the average energy of the laser beam during the recording of the dummy signal may be decreased stepwise or linearly by adjusting the duty ratio of the dummy signal stepwise or linearly; or otherwise the recording power may be varied. It is more preferred to vary the duty ratio of the dummy signal than to vary the recording power in order to simplify the apparatus. The dummy signal may be added to either the front or rear side of the recording information signal, that is mainly VFO, SYNC, and user data, depending on the recording media characteristics. In the preferred case of adding the dummy signal to only one side, the dummy signal is added to only the rear side of the recorded information signal rather than the front side where the VFO has a function of the dummy signal.
The distortion level of the reproduced signal waveform can also be lowered by shifting the start position of writing the recorded information signal to effect overwriting. In this case, the maximum shift width is preferably set to be shorter than the dummy signal that is added; particularly, it is preferably set to be approximately {fraction (1/20)} to xc2xd of the length of the recorded dummy signal.
Also according to the present invention the recording medium stores thereon information concerning the pattern of the dummy signal that is used, for example the duty cycle of the dummy signal. For example, it is preferred that such information be recorded in the control data area of the recording medium.
Recording films utilizing a phase change such as recording films of a Gexe2x80x94Sbxe2x80x94Te system and a Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te system may be used according to the present invention. The use of a recording film containing a high-melting point material such as Crxe2x80x94Te and Agxe2x80x94Te whose melting point is higher than that of the main component material and a recording medium with an Si/metal double reflective layer is preferred because the recording film thickness is further restrained from varying because of fluidization.
Still further, according to the present invention, information is recorded by an information recording apparatus including: a laser beam source, an optical system, automatic focusing (AF) means, tracking means, reproduced signal detection means, means to read information in a control data area, a recording medium on which information on a pattern of a dummy signal to be added to at least one of the leading or trailing sides of a recorded information signal is recorded beforehand, means for rotating or moving the recording medium, means for condensing laser beams from the laser beam source on the recording medium, signal modulating means for converting a signal to be recorded into a modulation code, means for adding the dummy signal as set forth herein, recording waveform generation means for generating a recording waveform corresponding to the recording code with the dummy signal added, recording start position control means for shifting a recording start position at random (preferably wherein the maximum shift width of the recording start position, for example, expressed in a unit of Bytes is shorter than the length of the dummy signal expressed in a unit of Bytes), laser driving means for driving the laser beam source in accordance with the recording waveform, means for converting the intensity change of the laser beam reflected from the recording medium into an electrical signal, means for amplifying the reproduced electrical signal, binary conversion means for converting the electrical signal into a binary waveform, means for deleting the dummy signal, and means for decoding the binary signal to make the decoded signal an information signal (user data).